A year after the massacre at Sandy Hook Elementary School, one of the city’s most prominent billboards now features a new grim tally — the approximate number of Americans killed by guns since that day: 32,833, the population of a small city.

John Rosenthal, the founder of Stop Handgun Violence, the group that sponsors the billboard, said the total will rise by 83 each day, reflecting national estimates of gun-related deaths. Of that number, eight are children, he said.

“That’s a Newtown classroom every three days in America,” he said, as he looked at the updated billboard along the Massachusetts Turnpike near Fenway Park. “Eight children killed, every day.”

Under the total runs the message “Shame on us all.” Rosenthal said he hopes the billboard will serve as a stark reminder of the scope of gun violence and urge people to lobby for stricter gun-control measures.

David L. Ryan/Globe Staff

“I hope people think ‘We have some responsibility here,’” said John Rosenthal, whose group sponsors the billboard.

Get Fast Forward in your inbox:

Forget yesterday's news. Get what you need today in this early-morning email.

Given the influence of gun rights advocates, Rosenthal said, he was not surprised by the failure of new federal gun control measures, including expanded background checks and a ban on assault weapons. More than two dozen states have weakened gun control laws since Newtown, where Adam Lanza killed 20 students and six staff members.

But Rosenthal said he remained hopeful that public pressure could force Congress to enact stricter laws.

A spokesman for the National Rifle Association could not be reached for comment Friday.

A new Associated Press-GfK poll found that a narrow majority — 52 percent — of Americans favor stricter gun laws. That marked a drop in support from January, when 58 percent backed stricter gun laws.

After the Newtown shootings, the Mass Pike billboard was redesigned to include 20 small, painted hands, symbolizing the slain schoolchildren. This week, organizers added the new message about the number of people killed.

The group has maintained the billboard since 1995. Tens of thousands of commuters see it every day.

Rosenthal praised Massachusetts gun laws as a model but said looser regulations in other states make it easy to bring firearms into the state.

The Newtown shootings have prompted a push for new gun control restrictions in Massachusetts, including limits on gun purchases and on capacity for ammunition magazines. But the pace has been slow.

In March, House Speaker Robert DeLeo established an advisory group to study ways to strengthen the state’s gun laws. That group, led by Northeastern University researcher and associate dean Jack McDevitt, has met regularly in recent months and said Friday it will submit its recommendations by year’s end.

“We took a comprehensive look” at ways to reduce gun violence, McDevitt said.

He said the panel, which included law enforcement officials, educators, and clinicians, met with a variety of groups and surveyed research about gun use.

McDevitt said the group would seek to prevent disturbed individuals from obtaining weapons without discouraging them from seeking treatment. Studies have not shown a link between mental illness and increased use of firearms, he noted.

Massachusetts, he said, has low gun ownership rates and a far lower gun homicide rate than most of the rest of the country. Still, there were 122 gun homicides in this state in 2011, McDevitt said.

“That number is way too high,” he said. “Our comparison shouldn’t be with other states, but with other industrialized nations” with lower rates.

In meetings with gun owners, the panel heard complaints that the process of obtaining gun licenses is arbitrary and leaves too much discretion to local police chiefs. Law enforcement officials called for a broader look at the issue, saying gun violence is often traced to dysfunctional families.

“They recommended a holistic approach,” he said. “They believe we need to look at this in a different way.”

McDevitt said that while gun violence in schools is extremely rare, mass shootings such as Newtown “have a huge effect on all of us.”